A laminated glass is a glass formed by bonding two glass sheets with a plastic interlayer made of e.g. PVB (polyvinyl butyral) interposed between them, and such a laminated glass is used as a windshield as well as a door glass of an automobile. This type of laminated glass is formed into a curved shape from the viewpoint of a body line and design of an automobile.
As the method for bending a glass sheet to be employed for a laminated glass, there is a method of placing a flat-plate shaped glass sheet on a forming mold having a bending-forming surface corresponding to a desired curved shape, and the forming mold is conveyed into a heating furnace, thereby to heat the glass sheet into the vicinity of the glass-softening point in the heating furnace. In this forming method, since the glass sheet is softened to be bent along the bending-forming surface of the forming mold by gravity, a glass sheet having a desired curved surface is produced. Further, as another bending method, a method of pressing a glass sheet heated and placed on a forming mold from upper side by a pressing means to bend the glass sheet, is also known.
A laminated glass for an automobile is fixed as it is fitted to a frame of an automobile, and at this time, in order to avoid breakage of the laminated glass, a plane compressive stress (hereinafter in this specification, a plane compressive stress formed at the edge of a glass sheet is referred to as edge compression, and it is abbreviated to as E/C) is formed at the edge of the glass sheet. In the glass sheet in which a residual stress is formed, a surface compressive stress is formed on the surface and an inner tensile stress is formed inside in the cross-sectional direction of the glass sheet. The plane residual stress is defined as follows. Namely, it is an integral value of the surface compressive stress and the inner tensile stress integrated along the cross-sectional direction of a glass sheet, and when the surface compressive stress is larger, the plane residual stress becomes a plane compressive stress. A region adjacent to the region of plane compressive stress, becomes a region of plane tensile stress wherein inner tensile stress is larger so as to balance with the plane compressive stress. In a region right inside from the edge, in order to balance with E/C, a plane tensile stress (hereinafter, in this specification, a plane compressive stress formed in a region just inside the edge of a glass sheet is referred to as inner tension, and it is hereinafter abbreviated to as I/T) is formed along the edge. This I/T has a peak in a peripheral region within about 50 mm inside from the edge of the glass sheet. When E/C is large, of course I/T is also large. A large plane tensile stress indicates that the plane tensile stress layer in this portion of the glass sheet is thin in the cross-sectional direction, and accordingly, the peripheral portion is a portion that tends to be destroyed as compared with the edge or the internal portion.
In a conventional laminated glass, since the edge and the peripheral portion of the glass sheet is covered with e.g. a mole made of a resin, there has not been a problem even if a certain degree of large I/T is formed. However, in a flash mount structure (a structure of fixing a laminated glass so that the body surface and the glass surface share a substantially the same plane) that is required as an automobile design, since the peripheral portion is exposed to the car-exterior side, it is required to reduce I/T.
Further, in recent years, from a demand for reducing weight of an automobile or from the viewpoint of safety of a passenger at a time of collision, component glass sheets each having a relatively small thickness of from about 1.5 to 3.2 mm are employed for a laminated glass. In order to fit such thin glass sheets to an automobile body by using a flash mount structure without breaking the glass sheets, it is necessary to prepare glass sheets having sufficiently large E/C and sufficiently small I/T.
Patent Document 1 being a prior art document discloses a glass sheet having a thickness of from 1.5 to 2.5 mm, which is a glass sheet for a laminated glass, wherein an E/C of from 24.5 MPa to 49.0 MPa is formed in the peripheral portion within 1.5 cm from the edge of the glass sheet. Since this glass sheet has a large E/C, the glass sheet inevitably has a large I/T. Accordingly, when the glass is designed so that the portion wherein I/T is formed is exposed to the car-exterior side, the glass tends to be fragmented by e.g. a flying stone.
Patent Document 2 discloses a glass sheet for a laminated glass, which is a glass sheet having a thickness of from 1.5 to 2.5 mm, wherein an E/C of from 19.6 MPa to 34.3 MPa is formed in the peripheral portion within 1.5 cm from the edge of the glass sheet, and an I/T of at most 7.8 MPa is formed in the internal region adjacent to the peripheral portion. Since this glass sheet has a large E/C and a large I/T, there is the same problem as that of Patent Document 1.
Patent Document 3 discloses a glass sheet for a laminated glass, wherein an E/C of from 4.9 MPa to 49.0 MPa is formed in the peripheral portion of the glass sheet. However, the document is silent as to I/T.
Patent Document 4 discloses a glass sheet for a laminated glass, which is a glass sheet having a thickness of from 1.5 to 4 mm, wherein an E/C of from 50 MPa to 100 MPa is formed in the peripheral portion of the glass sheet, and an I/T of smaller than 10 MPa is formed in an internal region adjacent to the peripheral portion. Since this glass sheet has a large E/C, it has a large I/T and there is the same problem as that of Patent Document 1.
Patent Document 5 discloses a glass sheet for a laminated glass wherein the maximum value of E/C is larger than 29.4 MPa and a glass sheet for a laminated glass wherein the maximum value of I/T is smaller than 3.9 MPa. Since this glass sheet also has a large E/C, the glass sheet has a large I/T, and there is the same problem as that of Patent Document 1.
Patent Document 6 discloses a glass sheet for a laminated glass, which is a glass sheet having a thickness of from 1.1 to 2.6 mm, wherein an E/C of from 20 MPa to 80 MPa is formed in the periphery of the glass sheet, and an I/T of from 0 to 15 MPa is formed its inside region. Since this glass sheet also has a high E/C, it has also a high I/T, and there is the same problem as that of Patent Document 1.
Patent Document 7 discloses a glass sheet wherein the plane residual stress monotonously changes from compressive stress to tensile stress from the peripheral portion of the glass sheet toward the central portion of the glass sheet, an E/C of at least 15 MPa is formed in the peripheral portion of the glass sheet, and a plane tensile stress of at most 2.5 MPa having no peak is formed in a region inside from the peripheral portion. Namely, in this glass sheet, no UT is present, and plane tensile stress is formed in the entire central region of the glass sheet. Since there is no I/T, it is possible to avoid a problem that a surface compressive stress layer becomes thin, but the production method is applicable only to a glass sheet such as a door glass of an automobile having a curvature in one direction.